Refactor Default System Exposure Values into an includable

[Copilot]

Refactors the "Default System Exposure Values" tip from docs/howto/bootstrap/collect.md into a reusable includable file, following the pattern established in the repository for similar content reuse.

Changes

• Created docs/_includes/default_system_exposure_values.md containing the tip with guidance on default System Exposure values
• Updated docs/howto/bootstrap/collect.md to replace the inline tip with an include-markdown reference
• Added the tip to docs/reference/decision_points/system_exposure.md (decision point reference page)
• Added the tip to docs/howto/gathering_info/system_exposure.md (gathering info page)

Benefits

This refactoring ensures consistency across the documentation by:

• Providing the same default value guidance in multiple relevant locations
• Maintaining a single source of truth for this content
• Making future updates easier by only requiring changes in one place

The implementation follows the same pattern used for other includable content in the repository (e.g., automatable_cvss_ssvc.md, safety_cvss_ssvc.md).

Closes #[issue_number]

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Refactor default system exposure values in collect.md into an includable</issue_title>
<issue_description>Child of #522

• Refactor the Default System Exposure Values tip from docs/howto/bootstrap/collect.md into an includable file in docs/_includes
• Replace the tip in collect.md with the included tip
• Include the tip to the decision point reference page docs/reference/decision_points/system_exposure.md
• Include the tip in the "gathering info" page for system exposure
  </issue_description>

<agent_instructions>This is very similar to the changes in PR #1010, follow those as an example.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #961

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Pull Request Overview

This PR refactors the "Default System Exposure Values" tip into a reusable includable file to improve documentation consistency and maintainability across the SSVC documentation.

• Created a reusable includable file for default system exposure values guidance
• Replaced inline tip content with include-markdown references in three documentation files
• Added the tip to two additional relevant pages for broader coverage

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
docs/_includes/default_system_exposure_values.md	New includable file containing the default system exposure values tip
docs/howto/bootstrap/collect.md	Replaced inline tip with include-markdown reference
docs/reference/decision_points/system_exposure.md	Added the tip via include-markdown
docs/howto/gathering_info/system_exposure.md	Added the tip via include-markdown

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